Aizen is god as well. There are too many "god" characters, so I wouldn't call that a good enough reason alone to call someone the most overpowered character.

Haruhi is not an action show, so overpowered really can't describe her. Madoka got the ability to defeat every witch ever, but that was explicitly granted to her and she can't merely use the power for whatever she wants.

Personally, going by comparison to the other characters in the series, Acnologia/Zeref from Fairy Tail or Madara, Obito, or the Juubi from Naruto. (If you're going for overpowered and unreasonable, shounen jump is the place to look) Kayaba Akihiko from SAO has his almost indefinite capacity for hacking also (immortal object).

Really it's just up to personal preference, and it's impossible to calculate the amount of fools who have already pointlessly debated this matter to no avail since there is no possible way of comparing them.

Will I get murdered for nominating The Hulk? I mean... Considering the fact that Marvel or rather... Disney already took a bite off of the anime goodness with their "X-men" "Wolverine" and "Blade" anime inspired series, it probably won't be long before they make more.

Even so, I don't believe The Hulk ever has been revealed to have a weakness and it frankly pisses me off when characters are made immortal -____- Naruto is the Hulk of Anime at the moment so it seems lol.

Oh by the way, if you didn't know about the Anime's from Marvel, don't bother... They look awesome, but are horribly empty and without soul entirely.

Alexander the Great, or "RIder" from Fate/Zero. His reality marble ability was pretty darn impressive. A portable bubble that opens a dimension to the great desert where a million warriors wait to battle!

And of course ANY character that can blow up planets. C'mon it's a bit silly.